Gaseous discharge lamp circuit



June 6, 1961 D. F. CHAPMAN GASEOUS DISCHARGE LAMP CIRCUIT Filed Sept.24, 1959 I 0 OOOUUUCWUUOUUOOOg i '4 2 Sheets-Sheet 1 l 00 MWWWOWWWO 0 42 3 N u L 2 4 0 UOUOUOOOOOOOOOWOO 00 INVENTOR Denim. FRcoK. CHAPm/w ATTO/ZNE Y June 6, 1961 D. F. CHAPMAN 2,987,650

GASEOUS DISCHARGE LAMP CIRCUIT Filed Sept. 24, 1959 2 Sheets-Sheet 2 N LN.. L

I I /I i i 3 34 B6 3A B J 1 w' L 12 3 4 oooooooooomoooo 000 4r F2 lA/VENTok Domm FREDK. HAPMAN ATTORNEY 7 inventionrelates tolcircuitsforlstarting and op- ..UP, F- .5 t Q crating one or more gaseousdischarge devices, such as fluorescent lamps, from an alternatingcurrent supply, such 1 lamps being provided with heatable electrodes,usually 1 filamentary electrodes, at each end. It known that the voltagerequired to start such lamps is much higher than the vvoltage requiredfor operation. .7

To reduce starting voltage it is well known .to provide initialpreheating for the lamp electrodes, such preheating causing emission of.electrons'and .ionisation of the gas filling and mercury vapour andthereby assisting starting. When the lamp has started it may bedesirable to reduce or cut off the preheating current supplied to thelamp electrodes in order to reduce power losses and improve the life ofthe electrodes. Known circuits have used a transformer with primary andsecondary windings to provide a power supply to preheat the electrodm Inthe Quickstart circuit, for example, the primary winding is connectedbetween opposite ends of the lamp and two secondary windings areconnccted across the two electrodes respectively. With this circuit achoke ballast is used in series with the supply to ensure stability andlimitation of lamp discharge current, and a capacitor is often connectedacross the supply to improve thelow power factor introduced by thechoke.

With such a circuit it hasbeen found that lamp starting may be diflicultif the ambient temperature is low or if atmospheric 7 humidity is high.To overcome these starting difiiculties it has been proposed to use lowgas filling pressures inside the lamps and to-coat the glass lampsurface with water-repellent materials. Such modifications to the lampmay increase cost and reduce life.

It is the objector, the present invention to provide an improved circuitfor starting and operating gaseous discharge'lamps in which startingdifliculties are reduced.

According to the present invention there is provided a circuit forstarting and operating one or more gaseous discharge devices havingheated electrodes from an alternating current supply, comprising avoltage step-down transformer having a primary winding and at least twosecondary windings, the primary winding being connected in series with aballast choke between supply terminals, two heater current terminals forconnection to one lamp electrode being connected in series with one ofthe secondary windings and a capacitor between the supply terminals, andanother of the secondary windings being connected to two further heatercurrent terminals for connection to another lamp electrode.

The invention will be described, by way of example, with reference tothe accompanying drawing containing circuit diagrams of a number ofembodiments of the invention.

In FIG. 1 there is shown a fluorescent lamp F having filamentaryelectrodes A and B and an auto-transformer having terminals and taps 1,2, 3 and 4. The parts 12 and 34 of the auto-transformer constitutesecondary windings while the part 2.3 constitutes the primary winding. Aballast choke G is connected in series between the tap 3 and a supplyterminal L. The electrode A is connected in series with the secondarywinding 12 and acapacitorCbetweensupplyterminalsNandL. The

' electrode B is connected in'parallel with the When a main switch (notshown) connecting the terminals N and L to an AC. supply is closed, amomentary inrush of current occurs through the series circuit comprisingcapacitor C, winding 1-2, and lamp electrode A. This surge of currentoccurs because the capacitor is initially discharged and the surgepersists for only a few microseconds until the capacitor becomes fullycharged. The magnitude of the current surge which occurs when thecircuit is switched on depends upona number of factors including thevalue of the impedance presented by the winding'1-2 in series with thecapacitor C. The current surge may be as much as 1000 amps. peak in acircuit such as that of FIG. 1 whenconnected to a 240 volt 50 cycle A.C.supply.

The momentary surge of current induces a momentary high voltage acrossthe Winding 12. Since the windings 2-3 and 3-4 are electromagneticallycoupled to the Winding 12 by transformer action, high instantaneousvoltages are induced in the former. Because the winding 2-3 has manymore turns than the winding 1-2, the voltage across 2-3 is many timesthat across 1-4. The result is that a very high transient voltage isinduced across thewin'ding 23, and hence between the this time beforethe main discharge has started is, low.

when the circuit is switched on. Such a high voltage, which may approach2000 volts, although it does not ,start the main discharge in the lampbecause at this time the filaments A and B are cold, serves to ionisethe gas and vapour in the discharge space of the lamp and enables themain discharge to start at a lower voltage when the filaments haveheated up.

The two electrodes A and B now rapidly reach a high temperature andemission of electrons occurs to help ionise the gas and vapour stillfurther. When ionisation is sutficient the lamp will start and a heavycurrent then 7 flows through the discharge space between the electrodes.This current is limited -by the impedance of the choke G and a voltagedrop now occurs across this choke. Voltage between the ends of the lampis thus reduced after starting and since the primary winding 2-3 is alsoconnected across the lamp the voltage on the winding 23 also drops.

In practical circuits the supply voltage is arranged to be about twicethe voltage across the lamp during operation. Thus voltage on thewinding 23 drops by about half after the lamp has started. Since thewinding 3-4 is coupled to the winding 2-3, the voltage supplied to theelectrode B drops after the lamp has started, thereby reducing thecurrent and hence the power losses in this electrode.

When the lamp is operating, current through the electrode A is thevector sum of two antiphase components, i.e. the current through thecapacitive branch and the current through the inductively controlleddischarge path. Since these currents are anti-phase they tend to canceleach other, with the result that after the lamp has started the currentthrough electrode A drops because of this partial cancellation as wellas through the reduction of voltage of the primary winding 2.--3.

During normal operation of the circuit of FIG. 1, the capacitor C servesto improve the power factor. Voltage across the capacitor C after thelamp has started can be shown to be the vector sum of three voltages,namely 'thesupply'voltage, the'voltage across 'the'winding 1-2, and thevoltage drop across the electrode A, and this capaoitonvoltage is foundto .be approximately equalto the supply voltage.

I By way of example, inone circuit as-shown inIFIG. lthe'lamp F. was aft. 8,0watt, typ eMCF/U. The

capacitor C was 0157.5 microfaradsjand the'choke G ga, impedance of 210"ohms at'0.85 arnp., '50 'c./s. The

winding 2'3 was of 1900turns 38 SWG enamelled co per Wire, eachsecondar'y winding was of 135 T ,tumslZflSWG enamelled copper wire, andthe'core'was a f'istack1of No'. 182;N Iaminations' punched from dy- Thefollowing values were obtained after switchingon and before the lampstarted and after the lamp had The circuits of FIGS. 2- to S idifi'eronly .inminor 'reispects from that of FIG. '1.

Thus the connections of 'the electrode A and the capacitor C to thewinding 1'-2 are rearranged and the choke G is connected to tap 4 in--=stead:of to tap 3*. The mode of operation is substantially the same aswith 'FIG. 1. shown as having tappings in order to permitoperation ondifferent supply voltages. Moreover a resistor R is shown connected inparallel with the capacitor C in order :In FIG. 5 the'choke G isto-ensurerapid discharge of the capacitor when .the cirvcuit is switchedoff. 'FA tapped choke G and azresis-tor-R may, of course, be used inanyof the circuits.

In FIG. 6 the only change from FIG. :1 is that the choke Gin FIG. 6 isconnected to a further tap 5'. This enables the steady voltage acrossthe lamp during starting to be made substantially greater than thesupply voltage. 1 Such a. circuitis, for example, suitable 'for use withan 8 ft. 125 watt fluorescent lamp which requires approximately 350volts for. satisfactory starting.

InF-IG. 7 the transformer winding is extended beyond the point 4 to apoint 6 to which the supply terminal L is connected through the'choke G.This enables the steady voltage across the lamp durin'g starting to bereduced considerably below the supply voltage. This may be advantageousfor short fluorescent lamps such as the 2 ft. watt lamp which requiresonly-about 110 volts for starting.

FIG; 8 shows how the oircuiti ofiFIG; 1 loan zbe modified tooperatei'two fluorescent-lamps F land F2 in series.

ILLFIG. 8 the filaments A andiB -of the lamps; F and F respectively aresupplied in the same way as thefilaments A- and '-B respectively in FIGs1;; the filaments B and A are supplied by'i'a' further secondary windingS on the transformer.

The other .circuitscan, of .coursc, be similarly adapted to operate aplurality of lamps.

. p 1.1A circuit for starting and operating at least .one'g'aseoushischarge device having, heated-electrodes from ,I claim:

an alternating current supply,c'omprising a voltage step- 1 downtransformer having aprimary winding: and at least "two secondarywindings; a ballastlchoke,.afcapacitor, sup- 7 7 'ply terminals for.coimectibnto a source .of alternating current, first and secondpairsko'f heater terminals each pair for connection to a lampelectrodeQmeanscOnnecting saidprimarywinding in series said ,ballastchoke between, said. supply terminals, .means connecting said first Ipair of heaterterminalsin sexiesiwithone -o f said secondary"windingsand saidcapacitonbetween said supply terminals, and meansconnectingsaidsecond'pair of heater go ten ninals across. another of saidsecondary. winding, one of said secondary windings-and said first pairof wherein said primarywindingl said ballast choke, at least heaterterminals are connected inseries between said supply'terminals', andmeans connecting at least oneterminal of each pair of heater terminalstoone of said alterj hating current supply terminals.

.2. [A circuit according to claim .1, wherein at least one .oftsaidsecondary windings is constituted by a part of said primarywinding.

3. A circuit according to claim l, .further comprising a resistorconnected in parallel with said capacitor.

References Cited in thefile of this patent UNITED STATES PATENTS2,294,623 Le Brun 'Sept. 1, 1942 2,444,408 1 Larime June 29 19482,824,262 Cates Feb. 18, 1958 2,838,714 Moerkens June '10, 19582,901,653 Gilmore Aug. 25, 1959

